Objectives: [Cryptococcus neoformans (Cneo) is the leading cause of fatal fungal infection in AIDS patients and the second most common fungal infection in transplant patients. In surviving patients, failure to eradicate Cneo, or similar pulmonary pathogens, often results in persistent infection associated with chronic lung damage. Current therapies are lengthy, have limited efficacy, and are often toxic. Impaired or aberrant host immune responses are a prerequisite for failed pathogen clearance and contribute to the lung damage associated with persistent infections. Yet these responses are poorly understood.] The present studies are designed to determine how immunomodulatory monocyte-derived lung dendritic cells (i-moDC) orchestrate the cellular and molecular networks which impair Cneo clearance and promote persistent pulmonary infections and chronic lung inflammation. [Improved understanding of these aberrant immune responses may yield novel therapies for the treatment of pulmonary infections caused by Cneo or related pathogens.] Hypothesis: [Susceptibility and persistence in response to cryptococcal lung infection] results from the aberrant development of i-moDC signaling through pathways involving IL-10, alternative activation gene products, and PD-1/PD-L1. The following aims have been constructed to rigorously test this hypothesis. Aim 1: To determine whether autocrine monocyte-derived DC IL-10 production is necessary and sufficient to up-regulate expression of alternatively-activated genes and PD-L1/PD-L2 in response to cryptococcal antigens (in vitro) or active fungal lung infection (in vivo). Aim 2: To determine whether pulmonary i-moDC promote the polarization and expansion of T regulatory cells [in susceptible mice that develop persistent lung infection.] Aim 3: To determine whether blockade of IL-10 or the PD-1/PDL-1 signaling pathways [improves fungal clearance and] prevents persistent cryptococcal lung infection. Research Plan and Methods: Our proposal utilizes a well-established murine model of cryptococcal lung infection in susceptible C57BL/6 mice characterized by an accumulation of immunomodulatory dendritic cells, an expansion in T regulatory and Th2 cells, and persistent infection associated with immune-mediated lung damage. The contribution of lung dendritic cells to [failed fungal clearance and] the persistently-infected phenotype will be determined using the following experimental techniques: 1) comparative flow cytometric analysis and cell sorting, 2) qPCR performed on isolated lung leukocytes, 3) fungal CFU analysis, 4) histology, 5) in vitro cell culture and co-culture, and 6) in vivo adoptive cell transfers. Our investigations will be enhanced by our selected use of: a) IL-10 deficient mice, b) IL-10 reporter mice (Vert-X mice), c) F1 generation mice (obtained from breeding C57BL/6 mice and BALB/c mice), d) transgenic mice containing the diphtheria toxin receptor permitting cell-specific depletion following diphtheria toxin administration, e) transgenic mice containing a floxed IL-10 gene permitting cell specific IL-10 deficiency, and f) administration of anti-IL-10 and anti-PD-L1 receptor blocking antibodies to disrupt immunomodulatory signaling networks [and improve fungal clearance.]